1. Field of the Invention
This invention relates to an improved resin sealing apparatus for use in manufacturing a resin-sealed semiconductor device.
2. Description of the Related Art
Resin sealing is one of methods of sealing semiconductor chips. There are known various resin sealing methods, among which is the transfer mold method. Since the transfer mold method ensures mass production of semiconductor chips at low cost, it is the most popular resin sealing methods at present.
FIG. 1 shows a mold of a conventional pot-type transfer mold apparatus, and FIG. 2 is a longitudinal cross-sectional view of the apparatus of FIG. 1. Molds 11A and 11B are separable upward and downward. Mold 11A has pot 13 at its center, in which resin is injected, and cavities communicating with pot 13 and having a shape corresponding to that of the semiconductor device package. Resin, material to be molded, is injected into pot 13, pressed by plunger 14 inserted in pot 13 to a predetermined pressure, and supplied into each cavity 12.
FIG. 3 shows a layout of mold 11B, in which the same portions shown in FIGS. 1 and 2 are identified by the same numerals as are used in FIGS. 1 and 2. Runners 15 or resin supplying paths are formed between pot 13 on the one hand and each cavity 12 on the other hand. Semiconductor chip 17 provided with a predetermined wire bonding is put on lead frame 16 and is arranged in each cavity 12.
In the apparatus as described above, in which mold 11B is arranged on mold 11A, when resin is injected into pot 13 and pressed by plunger 14 it is injected through runners 15 into each cavity 12, thus sealing the semiconductor chip 17 set therein. While a predetermined pressure is being applied to the resin, molds 11A and 11B are heated to 150.degree. to 190.degree. C., thermally setting the resin. Then, molds 11A and 11B are opened, and the semiconductor devices sealed with the resin are removed from the molds. In this way, a resin mold process is completed.
The resin mold process has the following drawback that, when resin is injected, air existing in hollow portions, such as pot 13, runner 15 and cavities 12, is entrapped. More specifically, as resin is supplied from pot 13 through runner 15 into cavities 12, the air existing in the hollow portions enters the resin, inevitably forming pin holes in the surface of the semiconductor device package. The holes impair the appearance of the device. Moreover, the holes may adversely affect the subsequent manufacturing steps, decreasing the production efficiency and the reliability of the product.
FIGS. 4A to 4C show products formed by the abovedescribed process. Resin 19 is supplied from pot 13 through runner 15 to cavities 12. The resin in pot 13 is called cull 13A. It is desirable that no holes be formed as shown in FIG. 4A in the product (package) made of resin 19 and located in cavities 12. However, in general, pin holes 18 are formed in resin 19 in cavities 12 as shown in FIGS. 4B and 4C.
To reduce holes 18 the following measures have been taken hitherto: to adjust the speed of injecting resin; to adjust the pressure applied to the resin after the injection; to provide air vents to cavities; and to heat the mold at a low temperature. However, these measures have drawbacks, as will be described below.
If the speed of injecting the resin is too low, the resin may be hardened before it fills all cavities. If the pressure on resin is too high, the resin may exude from a small gap of the mold, hindering the succeeding manufacturing steps. If the speed of injecting the resin is too high, or the pressure on the resin is too low, a great number of holes may be formed in the product.
Air vents, if provided, must be very small, particularly in a micro-semiconductor device. Such small vents may become clogged with resin, disabling the air vents to function. While the resin can be removed from he air vents each time the vents become clogged, the operating efficiency of the apparatus will, however, be lowered as a result. Hence, it is not practical to remove the resin from the air vents during consecutive operations.
If the mold is heated at low temperature, the resin quality may be degraded or else the reliability of the semiconductor device may be reduced. Hence, it is not advisable to heat the mold at a low temperature.
In addition, if the diameter of the cull and the capacity of the pot are both large, or the runners are long, air may easily enter the resin, and form holes in the molded packages. Further, if the runner has an angular cross section, the air existing in a corner of the runner enters into cavities when resin is pressed into the cavities, and forms holes in the resin in the cavities.